Currently, as interfaces for external storage devices, low-cost interfaces such as ATA (AT Attachment) and ATAPI (AT Attachment Packet Interface) are widely used. In an ATA/ATAPI, a host device regularly monitors a state of an external storage device, an event generated in the external storage device and the like by polling.
Also, in an ATA/ATAPI, the external storage device shifts to a low power consumption mode basically in response to an instruction from a host device. This is because an ATA/ATAPI interface does not have the function of notifying an event generated in an external storage device from the external storage device to a host device in an asynchronous manner and the external storage device can not shift to a low power consumption mode at its own judgment. If an external storage device shifts to a low power consumption mode at its own judgment, the external storage device can not receive a packet command from a host device and thus can not be reset to a normal operation mode according to an instruction from the host device. Therefore, in order to realize effective power management for an external storage device including an ATA/ATAPI interface, problems on how an instruction from a host device is given to an external storage device in a low power consumption mode and how an external storage device is reset to a normal state have to be solved.
To solve the above-described problems, conventionally, when an ATA/ATAPI interface circuit is in a standby state or a sleep state, a command-packet automatically-receiving-sequence circuit in the ATA/ATAPI interface circuit receives a packet command from a host device and generates a sequence signal for receiving command packet data (see, for example, Patent Reference 1). Moreover, when an external storage device (optical disk device) is half stopped, a reset signal asserted by the host device is extracted from an ATAPI bus and, based on the asserted signal, the optical disk device is reset to a normal state (see for example, Patent Reference 2).
In recent years, as an interface for external storage devices, a Serial ATA interface to is becoming widely used. The Serial ATA standard is obtained by changing a parallel transfer method employed in the ATA/ATAPI standard into a serial transfer method and, according to the Serial ATA standard, data transfer is performed using a small amplitude difference signal. Thus, compared to an ATA/ATAPI interface, the Serial ATA standard provides excellent noise resistance and is suitable for increase in speed. Furthermore, the Serial ATA standard includes the function of power consumption control by an external storage device itself, the AN (asynchronous notification) function of asynchronously notifying a state of an external storage device from the external storage device to a host device and the like.    Patent Reference 1: Japanese Laid-Open Publication No. 2001-184304    Patent Reference 2: Japanese Laid-Open Publication No. 2002-318646